means To stretch, push, or work something (or yourself) so hard that it bends, tears, or aches under the pressure — or the tension and injury that results.

from From Old French 'estreindre,' to bind or squeeze tight, which goes back to Latin 'stringere' — to draw tight, bind, or press. That same Latin root tightens its grip across English: it's a cousin of 'stringent,' 'strict,' and 'constrain,' all words about things pulled taut to the point of complaint. The 'lineage' sense of strain — as in a strain of bacteria or a noble strain — is a separate thread entirely, from Old English 'strēon' meaning 'gain' or 'begetting,' which only later got tangled up with this one.